An electronic component is generally soldered onto printed circuit boards in one of two ways. Using a first approach, the component is mounted to a surface on one or both sides of the printed circuit board (known as "surface mount" technology). Alternatively, the component is mounted to the printed circuit board by mounting the body of the component on one side of the printed circuit board, inserting leads through holes in the board plated with a conductor (such as copper), and applying solder to the leads on the opposite side of the board (known as "through hole" technology). Through hole technology is well illustrated in FIG. 1 which shows a cutaway view of a single lead 1 inserted through a through hole 2 covered with copper laminated pad 3 in printed circuit board 10 (made of fiberglass/phenolic) and solder 4 holding lead 1. This patent application relates to both types of mounting in which the "bottom" side of the board will be "wave soldered".
Automated wave soldering systems for wave soldering of through hole components and surface mount components include a series of apparatus or stations within the system, each of which performs one step in the soldering process. The soldering process begins with the application of flux and/or other fluid used in preparation for soldering onto a printed circuit board, followed by the preheating of the board and the application of solder to the leads (in "through hole" technology) or component (in "surface mount" technology) protruding from the bottom of the board. The board is transported in the wave soldering system along a conveyor.
A significant problem with this prior art automated process has been that in some instances a bridge of solder has formed between adjacent conductors causing a short circuit. In through hole technology (FIG. 1), upon application of the solder to the bottom surface of the printed circuit board, strong adhesive forces bond the negatively-charged solder 4 which adheres by capillary forces between the wettable lead and positively-charged copper laminated pad 3 around through holes 2 and to the positively-charged copper leads 1 on the components passing through holes 2. In surface mount technology, the same type of forces bond the solder to surface mount component terminations. The solder molecules bond to each other with less strong cohesive forces. Therefore, excess solder between joints typically falls away and does not create a bridge. However, where joints are very close together, where leads are improperly clinched, or where the components mounted to the board have long leads which extend from the surface of the printed circuit board more than approximately 0.030 inches, or 0.8 mm, the strength of the adhesive forces may cause a bridge to form. Several processing-related factors related to wave dynamics, conveyor speed, conveyor angle, and/or using insufficient or no flux prior to soldering or using nitrogen during soldering either of which may increase surface tension, may also result in the formation of bridges.
FIG. 2A illustrates the bottom of a sample printed circuit board 10 before wave soldering. On this "mixed technology" board, to which this invention is by no means limited, some components are surface mounted and some components are mounted with through hole technology to form a printed wire assembly. Those components which are mounted with through holes to board 10 include D-Sub connectors 20, 21 which are mounted through through holes in board 10 and pin headers 23, 24. Components which are surface mounted to the bottom of illustrated board 10 include SOIC's (small outline integrated circuits) 30, 31, SOT's (small outline transistors) 33 and chip resistors 34-37. Also illustrated are large and small vias 25, 26, respectively, through which current is supplied to components on top of board 10 and mounting holes 22. The top of board 10 is illustrated in FIG. 2B.
FIG. 2C illustrates board 10 of FIG. 2A after wave soldering (and without any debridging). As shown, bridges 40, 41, 43 in board 10 have formed between terminations on SOIC 31, between chip resistors 34-36, and between chip resistor 37 and SOT 33, respectively.
FIG. 2D shows a cross-sectional view of board 10 of FIG. 2C along line D-D'. In addition to bridges 40, 41, 43, bridges 44, 45 have formed between leads 50, 51 on connector 21 and between leads 52, 53 on pin header 24 during wave soldering.
The cohesive forces in the solder strengthen as the solder solidifies. Thus, a bridge can be removed without damaging the joint only if the solder has not solidified. The time of solidification is dependent on several variables including the solder alloy used, the solder temperature, board mass, solder mass and environment temperature. One time consuming method of removing a bridge is to inspect each board for bridges after it exits from the mass soldering system, manually removing the bridge of existing solder with a soldering iron and/or soldering wick and manually resoldering the joint.
An alternative to manual debridging, currently in use in some commercial wave soldering systems, is to incorporate a hot air knife following the soldering station. This type of hot air knife is described in U.S. Pat. Nos. 4,401,253, 4,402,448, and Re. 32,982. The hot air knife shown in these patents (which is also implemented to some degree commercially by Electrovert Ltd., Grand Prairie, Tex.) comprises a manifold extending the entire width of the conveyor. The Electrovert hot air knife blows the entire width of the conveyor regardless of the width to which the conveyor is adjusted. Compressed air or inert gas is output from the orifice of the hot air knife at a high flow rate and is aimed at the bottom of the board following the application of the solder while the solder is still molten. While the adhesive and capillary forces retain the solder on the joints, the hot air knife breaks the cohesive forces and "relocates excess solder on, and/or blasts excess solder from the underside of the board, interconnections, leads, and bodies, and in doing so also minimizes the possibility of solder bridging or icicling or short formation upon solidification." Re. 32,982, col. 5, lines 48-52.
Electrovert's AccuKnife has both a vertical adjustment in the height of the nozzle and an adjustment of the nozzle towards or away from the wave, both requiring movement of the entire manifold.
Electrovert currently manufactures commercial products known as AccuKnife, a hot air debridging tool, and coN.sub.2 tour.sup.PLUS, a similar device which performs the debridging function using inert hot gas. Re. 32,982 describes the stream as heated to a temperature in the range of 93.degree. C.-350.degree. C. It is applicants' understanding that in Electrovert's commercial products the stream of air or inert gas must be heated to a temperature above the temperature of molten solder (known as the eutectic point of solder, which for the current standard 63/37 tin/lead solder alloy is 183.degree. C.).
Electrovert's present commercial hot air knives (including AccuKnife and CoN.sub.2 tour.sup.PLUS), and the hot air knife shown in U.S. Pat. Nos. 4,401,253, 4,402,448, and Re. 32,982 present significant disadvantages because they extend over the entire width of the conveyor system carrying the printed circuit board and operate continuously at a high flow rate when the soldering system is on. These hot air knives, therefore, unnecessarily treat the entire bottom surface of the board although many, if not most, of the joints do not require debridging. As a result, defective joints are induced and some weaker joints, including larger joints, such as large vias 25 or mounting holes 22, where cohesive forces of solder joints are weak, are blown out, thereby adversely affecting the reliability of the board. The boards have to be inspected and manually repaired after application of the hot air knife when this occurs. The prior art also creates additional work to clean the soldering equipment of undesirably blown out weak joints.
Moreover, the application of a large quantity of heated air required by the prior art further dictates that the manifold in which the air is heated be relatively large and therefore heavy, and consume large amounts of energy to compress and heat the air. Increased air conditioning is also needed to cool the room in which the soldering system is operated due to the amount of heat exhausted by the prior art system.
These disadvantages are particularly pronounced in the Electrovert AccuKnife in which the hot air knife always remains on and compressed air or gas is continuously output from the orifice of the hot air knife while the soldering system is activated.
The CoN.sub.2 tour.sup.PLUS suffers somewhat less from these disadvantages as the inert gas only flows at a rate required for debridging when a printed circuit board travelling through the conveyor system is above the hot air knife. When the printed circuit board is not above the hot air knife, the CoN.sub.2 tour.sup.PLUS emits a constant bleed stream at a low flow rate.